Hi PeterDavenport,
You could follow the steps below to add a domain to Microsoft 365, but please note that you must be a Global Administrator of a business or enterprise plan.
- Go to the admin center at https://admin.microsoft.com.
- Go to the Settings > Domains page.
- Select Add domain.
- Enter the name of the domain you want to add, then select Next.
- Verify your ownership of the domain.
- Change the DNS required for Office to use your domain.
- If you chose to add DNS records yourself , select Next and you'll see a page with all the records that you need to add to your registrars website to set up your domain.
For more information you could refer to: Add a domain to Microsoft 365